1. Field of the Invention
The present invention relates generally to the field of subterranean formation evaluation during well coring operations. More specifically, this invention relates to measurement while coring techniques for the real time in-situ measurement of the chemical and or physical properties of a core during coring operations.
2. Description of the Related Art
Coring operations occur during the drilling of oil, gas, and water wells to recover parts of the subterranean earth formation (a core) for analysis of the chemical properties, physical properties, and or fluid saturations of the core within the earth formation. The downhole assembly for a coring operation generally comprises a coring bit, an inner and outer core barrel, and one or more stabilizers that provide weight on the core bit and stability to the entire downhole assembly during operation. As the coring proceeds, the coring operator periodically brings the inner core barrel, which also serves as a container for the core downhole, to the surface (a "trip") to remove the core for analysis at the surface.
When bringing the core from the bottom of the well hole to the surface, significant pressure and temperature changes occur that result in gas expansion as well as the evolution of gas from the oil. During this de-pressurization (the bringing to the surface) the chemical and or the physical properties of the core including the fluid saturations undergo a substantial change, which means that the analysis of the core at the surface will have some amount of error because of this change in the core from its down hole state. Pressure coring and Sponge Barrel coring are techniques used in the past to avoid the above problems. However, the use of these techniques is somewhat rare due to the expense involved in their use.
Another technique is to add mudwater tracers to the drilling mud to account for changes in the fluid saturation of the core due to flushing by the mud filtrate, depressurization, and other processes. The addition of the mudwater tracers to the drilling mud occurs at the surface, which allows the tracers to "invade" the core during the coring process. When recovered at the surface, the analysis of the core requires radially sectioning of the core, extraction, and searching for the tracer to monitor the mudwater invasion. This analysis technique is both time consuming and expensive and still may not provide accurate measurements as noted above, especially if the core contains the three fluid phases present in it. Another disadvantage of this technique is that it does not correct for any changes in the porosity of the rock.
Due to the high cost of coring, most chemical and or physical information about the subterranean earth formation comes from wireline downhole well logging. This method for measurement gathering involves lowering a measurement device attached to a wire into the drilling hole.
One problem with this type of measurement technique is that the sensor source and the sensor detector are both inside the borehole, which results in sending the measurement signal out into the general subterranean earth formation measure where the signal will reflect and scatter. As a result only a small volume of the earth formation near the wellbore responds to the applied sensor source. Even then, signal artifacts due to the borehole rugosity may cause large errors in the measurements. Another problem with this technique is that it does not define or restrict the volume of the earth formation investigated by the sensor signal very well. And finally, the analysis of the measurements gathered by this technique requires many semi-empirical corrections (to the measurements) to account for the poorly defined geometry and other factors including mud filtrate invasion.
The present invention is an apparatus and method for measuring the downhole chemical and or physical properties of the core during the coring operation. The present invention accomplishes this by appropriately instrumenting the core barrel with a downhole measurement device that allows the in-situ and real time measurement of the chemical and or physical properties of the core such as the porosity, bulk density, mineralogy, and also the fluid saturations of the core. The present invention offers many advantages over the prior techniques including the ability to measure the in-situ saturations of oil, water, and gas that are not currently possible with the current techniques. Additionally, the present invention offers an advantage over wireline downhole well logging because the sensor signal travels through the core within the inner core barrel, which is a known geometry (the inner core bore) unlike the earth formation along the well hole, that causes the sensor signal to scatter or reflect. Another advantage of the present invention is that it completes most if not all of the measurement gathering of the core before the core reaches the surface, which minimizes the cost of analyzing the core after it is at the surface.